From McGraw-Hill's Engineering Companion

CHEMICAL ENGINEERING [*]

DIFFUSIONAL OPERATIONS

1 Mass-Transfer Fundamentals.

The transfer of material from one phase to another is a primary means of separating multicomponent solutions. In general, two equilibrium phases of a multicomponent mixture will have different chemical compositions, and this difference offers a means for separating a mixture into its individual components. Repetitive phase changes can provide increasingly pure solutions and in the limiting case can produce pure individual components. Analysis of these phase-change separation processes depends on three factors: thermodynamic equilibrium, mass-transfer rates, and pattern of contact between phases.

Mass-Transfer Rates The amount of contacting required to bring two phases into equilibrium is dependent on the rate of mass transfer. The rate at which mass is transferred between phases is controlled by the driving force for mass transfer, the resistance to mass transfer, and the interfacial area between phases, according to

The overall mass-transfer coefficients are dependent on resistance to mass transfer in interfacial films (in a manner analogous to film resistances in convective heat transfer), which depend on molecular parameters, fluid turbulence near the interface, and the equilibrium relationship between phases. For equilibrium relationships which are essentially the straight lines y = mx, mass-transfer coefficients are given by

The coefficients are ordinarily determined experimentally as volumetric coefficients K Ga and K La, since interfacial areas are difficult to determine.

Continuous Contacting. Continuous-contact processes may be run with concurrent, crosscurrent, or countercurrent flow patterns. In terms of...